Generally, in preparing liquid crystal display devices, liquid crystal cells comprising liquid crystals and polarizing plates are basically required and suitable adhesive layers or pressure-sensitive adhesive layers have to be used for binding them. In addition, for improving functions of liquid crystal display devices, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancement film, and the like may be used, with additionally adhered to the polarizing plate.
Major structure forming a liquid crystal display device comprises, generally, a uniformly aligned liquid crystal layer, a polarizing plate with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisted of a transparent glass plate or plastic sheet material containing a transparent electrode layer; a phase retardation plate; and an additional functional film layer and the like.
The structure of polarizing plate is one comprising an iodine compound or a dichroic polarizing material aligned in a certain direction. To protect these polarizing elements, multi-layers are formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizing plate may additionally comprise a phase retardation film, or a compensation film for wide view angle such as a liquid crystal type film, in a shape having a unidirectional molecular alignment.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under high temperature, or high temperature and humidity conditions, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient. As a result, if the polarizing plate is fixed by a pressure-sensitive adhesive, then stress is concentrated on the TAC layer by shrinkage or expansion under high temperature, or high temperature and humidity conditions, thereby birefringence is developed and light leakage occurs. In this case, a negative birefringence is usually caused over the layer by the shrunk TAC.
Meanwhile, the pressure-sensitive adhesive layer needs high cohesion strength at high temperature to maintain the endurance reliability, for which partially cross-linked visco-elastic materials are used. When the partially cross-linked structure is introduced into the pressure-sensitive layer, the pressure-sensitive layer has the residual stress under the given stress and the polymer in the cross-linked structure is aligned in the specific direction to develop birefringence. Under such alignment, general alkyl acrylic pressure-sensitive adhesives develop birefringence of negative values as in TAC.
Meanwhile, monitor size of computers, and the like, becomes larger, and the demand is recently rapidly increased for LCD TVs using polarizing plates. As such panels become larger, polarizing plates also become larger, and thereby the residual stresses of the layers of TAC and the pressure-sensitive adhesive becomes higher and thereby the negative birefringence is increased and thus light leakage is extremely increased.
Among methods of minimizing light leakage under sixth residual stresses, a method may be considered, such as a method that an overall birefringence, including a TAC layer and pressure-sensitive adhesive, under the residual stress is minimized by adding (blending) materials representing birefringence of positive values to the final pressure-sensitive adhesive layer or copolymerizing acrylic monomers having a positive birefringence.
KR laid-open patent publication No. 2003-0069461 discloses a pressure-sensitive adhesive correcting birefringence of negative values that the acrylic pressure-sensitive adhesive layer represents under the residual stress by incorporating 0.01 to 40 parts by weight of a low molecular weight material representing birefringence of positive values under the residual stress into the acrylic pressure-sensitive adhesive layer. However, in said pressure-sensitive adhesive, the modulus of pressure-sensitive adhesive is lowered, due to the incorporated low molecular weight material therein. Therefore, there is a problem in tailoring property on processing the polarizing plate. In addition, there are a problem of moving the low molecular weight material into the interface for long-term storage and the possibility of phase separation with the acrylic pressure-sensitive adhesive.
A method of minimizing birefringence by copolymerizing a monomer representing negative birefringence and a monomer representing positive birefringence under the residual stress is also known (refer to Applied Optics (1997)). In a specific example, the degree of birefringence may be regulated under the given stress by copolymerizing an acrylic monomer (negative birefringence) having a side chain of an alkyl group and an acrylic monomer (positive birefringence) having a side chain of an aromatic group.
JP Unexamined Patent Publication No. 2002-332468 describes a method of improving plastic resistance of the pressure-sensitive adhesive layer by introducing an acrylic monomer containing an aromatic group in a side chain. In addition, U.S. Pat. No. 6,663,978, and JP Unexamined Patent Publication Nos. 2002-173656 and 2003-013029 describe a method of regulating a refractive index of a pressure-sensitive adhesive layer by introducing an acrylic monomer containing an aromatic group in a side chain. Further, JP Unexamined Patent Publication No. 2005-053976 describes a method of improving adhesion performance in even low polar films by introducing an acrylic monomer containing an aromatic group in a side chain.